I am reading the collected works of Judy Blume for reasons I detail here. You're in luck: this is the first one. You haven't missed anything yet.
Poor Freddy. He's in the middle of an impressive older brother and an adorable younger sister. Every day of his life is something like Alexander's Terrible, Horrible, No Good, Very Bad day. He can't do anything right. We know how it is.

This is Blume's first book, and is a very simple picture book. My edition is 39 pages of both large text and illustration. Each chapter is perhaps 300 words. Basic plot: Freddy feels lonely and un-special as a middle child. He signs up for the school play because it's something his brother had never done, and despite being too little for a speaking part, is cast as a Green Kangaroo. The Green Kangaroo. He practices hopping around and does a great job, and basically finds his niche.

I was not a middle child, but I owned this book when I was young. I remember feeling connected to Freddy's need to do his own thing, and many years after I read it, I experienced a very similar theatrical awakening. What Blume does well here is what she always does well: she expresses the worries of kids in a respectful and believable way. She knows that if a seven year old were really cast as a green kangaroo, they'd have a busy two weeks practicing hopping. She knows that green freckles might be sort of overwhelming. She knows that this moment of self-reflection and of transformation is real for every actor, right before opening night: "He jumped over to the mirror. He looked at himself. He really felt like a Green Kangaroo."

Everything turns out great, of course, though we don't get the details of the play-- just what it was like to be Freddy within the Kangaroo. It ends, of course, on a happy note: Freddy was happy being Freddy. We knew it would go this way.

But look at this illustration (by Amy Aitken). Freddy's brother and sister are sliding into their own private jealousy. These are themes-- jealousy, rivalry, the ambiguity of sucess-- that Blume will return to again and again. Yes, this is her first and simplest, but there are hints of Judy throughout this story. Just wait until she puts these sorts of unreliable narrators in situations with much more social meaning and ambiguity.